1. Field of the Invention
The invention relates to a linear damper, particularly to one that has a driving rod displacing linearly for a circular rotation operation of a plurality of permanent magnets, so as to adjust a damping force of the driving rod by controlling an overlapped area of the permanent magnets and a magnetic surface, and an eddy load formed thereby.
2. Description of the Related Art
In Indoor sports or rehabilitation equipment, some actuating mechanism such as rowing fitness and strength training machines must have load device or damping device. A foot sports equipment 10 as show in FIG. 1A has a seat 11, two pedals 12 arranged on the seat 11 for stepping thereon, and a damping device 13 connected between the pedals 12 and the seat 11. Due to the limitation of space, the damping device 13 is mostly a hydraulic cylinder as shown in FIG. 1B in application to indoor sports equipment. Such hydraulic cylinder generally includes a cylinder 131, a piston 132 being installed in the cylinder 131 and having a passage 133 arranged thereon and a piston rod 134 at an end thereof, and a predetermined quantity of hydraulic oil 135 filled in the cylinder 131. When the piston rod 134 displaces linearly in the cylinder 131 subject to an axial force, the hydraulic oil 135 would pass through the passage 133 on the piston 132 to produce the damping effect.
The piston rod 134 of the conventional hydraulic damping device 13 has frequent frictions with an oil seal 136 due to the back and forth operation, resulting in an oil leakage problem fouling ground and equipment, and affecting the damping function of the device. Moreover, after a period of using the hydraulic oil, the viscosity of the hydraulic oil will change, and it is susceptible to high temperatures, causing an instability damping effect. Also, the hydraulic oil will have to flow from the piston to the other side during the operation; since the reaction speed of the exchanging path of the hydraulic oil is slow, the exercise equipment cannot be performed well.
The present inventor therefore has disclosed a linear damper in the U.S. patent application Ser. No. 14/105,829 as shown in FIG. 2. The main features of the linear damper is that a driving rod 30 is linearly displacing in a tube body 20, rotating a fixed seat 60 by having a screw sleeve 40 driving a screw rod 50 for rotation; then a plurality of permanent magnets 63 arranged on the fixed seat 60 would be operated in circle rotation, forming a torque by an eddy current produced between the permanent magnets 63 and a magnetic surface 73 of a cover 70, so as to provide a buffer and damping force. The subject invention has avoided problems of oil leakage and unstable damping effect, but the eddy current produced between the permanent magnets 63 and the magnetic surface 73 is unchanging; in other words, the device can only provide certain degree of the torque. However, for users of fitness equipment, an unchanging degree of damping force cannot meet the need of fitness training since different degree of the damping force is needed in different stages of the training. Therefore, the present inventor has been diligently working in a device with adjustable damping force for its users.